Alternative circulation cooling method for emergency core cooling system, and nuclear power plant

ABSTRACT

The present invention provides an alternative circulation cooling method for an emergency core cooling system that, even if the emergency core cooling system does not operate normally, can prevent the implementation of containment vessel venting by suppressing a rise in pressure and temperature in the containment vessel, and can suppress the implementation of dry-well venting even if containment vessel venting needs to be performed, as well as a nuclear power plant that is capable of the same. An alternative circulation cooling method for an emergency reactor core cooling system is performed at a nuclear power plant that includes an RHR system and a MUWC system. The method includes: connecting the downstream side of an RHR heat exchanger to the upstream side of a MUWC pump, and cooling water from a suppression chamber using the RHR heat exchanger and performing nuclear reactor injecting or containment vessel spraying using the MUWC pump.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2015-196741, filed on Oct. 2,2015, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an alternative circulation coolingmethod for an emergency core cooling system at a nuclear power plant,and a nuclear power plant that is suited to the implementation of thesame.

Background of the Invention

Boiling water reactor (BWR) nuclear power plants are provided with anemergency core cooling system (ECCS) for injecting a coolant into thecore in order to cool it if the coolant in the core has become depleted.The nuclear reactor injection equipment in an emergency core coolingsystem is a high pressure system and a low pressure system (e.g., see JPH08-114699A).

A high pressure system is a system that can inject a coolant into thereactor core at a pressure even higher than the pressure in the nuclearreactor (approximately 7 MPa) during a rated operation. Examples of highpressure systems include a high pressure core flooder system (HPCF) anda reactor core isolation cooling system (RCIC).

A low pressure system is a system for ensuring that water is injectedinto the reactor core after the pressure has been lowered by adepressurization system. Examples of low pressure systems include a lowpressure core spray system (LPCS) and a residual heat removal system(RHR). A residual heat removal system has various operation modes,examples of which include a containment vessel spray mode, a shutdowncooling mode (decay heat removal), and a suppression chamber coolingmode.

There are also alternative cooling injection systems that are providedfor the case where even the above-described emergency core coolingsystem cannot sufficiently cool the nuclear reactor (e.g., see JPH08-285983A). Examples of alternative cooling injection systems includea make-up water condensate system (MUWC), a fire protection system (FP),and the injection of water by a fire engine or the like. A make-up watercondensate system is originally a system for supplying condensate waterfrom a condensate storage tank to equipment in buildings or the like,but can also supply condensate water to the reactor core in anemergency. Note that condensate water is a coolant (water) that isobtained by using a main condenser to cool and condense steam thatturned a turbine.

JP H08-114699A proposes that when the emergency core cooling system isoperated using emergency power, the high pressure system pump and thelow pressure system pump are assigned to different power sections andoperated at the same time. The aforementioned document states that doingthis makes it possible to reduce the capacity of the emergency dieselgenerator. JP H08-285983A proposes the use of a steam-water separatorstorage pool instead of a condensate storage tank as the water sourcefor the emergency reactor core cooling equipment. The aforementioneddocument states that doing this makes it possible to reduce the capacityof the condensate storage tank and reduce the quality control class andearthquake resistance class to classes that are standard for regular-useequipment.

JP H08-114699A and JP H08-285983A are examples of related art.

As described above, nuclear power plants are provided with severalcooling apparatuses as emergency core cooling systems. They are alsoprovided with an alternative cooling injection system for accidentmanagement (AM) when the emergency core cooling systems do not functionnormally.

However, in consequence of the operation of conventional alternativecooling injection systems such as a make-up water condensate system or afire protection system, the amount of water in the containment vesselincreases due to the use of water from outside the containment vessel.If condensate water or the like continues to be injected, and the waterlevel in the suppression chamber (wet-well) surpasses the wet-wellventing line, wet-well venting cannot be performed. For this reason, theinjection of water from outside of the containment vessel is restrictedsuch that the wet-well venting line is not surpassed, and consequentlycontainment vessel venting is implemented if the pressure in thecontainment vessel rises. Wet-well venting is processing for releasing agas in the containment vessel to the outside through the water in thesuppression chamber (suppression pool water). This results in therelease of radioactive material that should originally be contained inthe containment vessel, and thus is processing that should be avoidedwhenever possible.

Also, if the amount of injected water surpasses the wet-well ventingline, dry-well venting is performed using the containment vessel vent.Dry-well venting is processing in which a gas in the reactor containmentvessel is discharged directly into the outside air. This gas does notpass through the suppression pool water, and thus has a large influenceon the outside. Specifically, if the alternative cooling injectionsystem operates for an extended period of time, radioactive materialwill be released to the outside due to containment vessel venting, andif the water level in the suppression chamber is not restricted,dry-well venting, which has a large influence on the outside, will beperformed.

In view of this, an object of the present invention is to provide analternative circulation cooling method for an emergency core coolingsystem that, even if the emergency core cooling system does not operatenormally, can prevent the implementation of containment vessel ventingby suppressing a rise in pressure and temperature in the containmentvessel, and can suppress the implementation of dry-well venting even ifcontainment vessel venting needs to be performed, as well as a nuclearpower plant that is capable of the same.

BRIEF SUMMARY OF THE INVENTION

In order to solve the issues described above, an alternative circulationcooling method for an emergency core cooling system according to arepresentative aspect of the present invention is an alternativecirculation cooling method for an emergency reactor core cooling systemthat is performed at a nuclear power plant that includes a residual heatremoval system (RHR) for removing fuel decay heat when a nuclear reactoris shut down and a make-up water condensate system (MUWC) that supplieswater from a condensate storage tank into the nuclear reactor, themethod including: connecting a downstream side of a heat exchanger ofthe residual heat removal system (RHR) to an upstream side of a MUWCpump of the make-up water condensate system (MUWC); and cooling waterfrom a suppression chamber using the heat exchanger of the residual heatremoval system (RHR) and performing nuclear reactor injecting orcontainment vessel spraying using the MUWC pump.

According to the above configuration, even if the emergency core coolingsystem (ECCS) that includes the residual heat removal system does notoperate normally, it is possible to suppress a rise in temperature andpressure in the reactor containment vessel by cooling water from thesuppression chamber and circulating it to the reactor core. For thisreason, it is possible to suppress the implementation of venting due toa rise in pressure, or significantly delay the implementation timing. Inparticular, the amount of water in the reactor containment vessel doesnot increase when cooling the reactor core, thus making it possible tosuppress a situation in which the water level in the suppression chamberrises and surpasses the venting line. Accordingly, even if venting needsto be performed, it is possible to ensure the implementation of wet-wellventing and avoid the implementation of dry-well venting.

In the method described above, the nuclear power plant may include ahigh pressure core flooder system (HPCF) that supplies water from thecondensate storage tank into the nuclear reactor, and the downstreamside of the heat exchanger of the residual heat removal system (RHR) maybe connected to the upstream side of the MUWC pump of the make-up watercondensate system (MUWC) using pipes of the high pressure core floodersystem (HPCF). According to this configuration, the downstream side ofthe heat exchanger of the residual heat removal system (RHR) and theupstream side of the MUWC pump of the make-up water condensate system(MUWC) can be connected without providing a new pipe.

A nuclear power plant according to a representative aspect of thepresent invention is a nuclear power plant including: a residual heatremoval system (RHR) for removing fuel decay heat when a nuclear reactoris shut down, and a make-up water condensate system (MUWC) that supplieswater from a condensate storage tank into the nuclear reactor, wherein abypass line that connects a downstream side of a heat exchanger of theresidual heat removal system (RHR) to an upstream side of a MUWC pump ofthe make-up water condensate system (MUWC) is provided, and whereinwater from a suppression chamber can be cooled using the heat exchangerof the residual heat removal system (RHR), and nuclear reactor injectingor containment vessel spraying can be performed using the MUWC pump.

According to the above configuration, even if the emergency core coolingsystem that includes the residual heat removal system does not operatenormally, it is possible to cool water from the suppression chamber andcirculate it to the reactor core. In particular, the downstream side ofthe heat exchanger of the residual heat removal system (RHR) isconnected to the upstream side of the MUWC pump of the make-up watercondensate system (MUWC) by the bypass line, and thus water does notflow through a pump or a check valve that are arranged on the line forthe high pressure core flooder system. Accordingly, there is no risk ofthe check valve closing, and pressure loss due to the high pressure coreflooder pump does not occur, thus making it possible to stably andreliably circulate water.

The method includes connecting the downstream side of an RHR heatexchanger to the upstream side of a MUWC pump, and cooling water from asuppression chamber using the RHR heat exchanger and performing nuclearreactor injecting or containment vessel spraying using the MUWC pump.

Advantageous Effects of Invention

According to the alternative circulation cooling method for an emergencycore cooling system and the nuclear power plant of the presentinvention, even if the emergency core cooling system does not operatenormally, it is possible to suppress a rise in temperature in thesuppression chamber and suppress dry-well venting.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram illustrating portions of a conventional nuclearpower plant, residual heat removal system, and high pressure coreflooder system that are related to the present invention.

FIG. 2 is a diagram illustrating a make-up water condensate system.

FIG. 3 is a diagram illustrating a first embodiment.

FIG. 4 is a diagram illustrating a second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention will be described indetail below with reference to the accompanying drawings. Thedimensions, materials, other specific numerical values, and the like inthis embodiment are merely illustrative examples for facilitatingunderstanding of the invention, and are not intended to limit thepresent invention unless otherwise specifically stated. Note thatelements having substantially the same functions and configurations inthis description and the drawings have been given the same referencesigns in order to omit redundant descriptions, and elements not directlyrelated to the present invention have been omitted from the drawings anddescriptions.

FIG. 1 is a diagram illustrating portions of a conventional nuclearpower plant, residual heat removal system, and high pressure coreflooder system that are related to the present invention. A reactorcontainment vessel 100 is a piece of equipment that is made ofreinforced concrete, and accommodates a reactor pressure vessel 110 inits interior space. The reactor pressure vessel 110 is a piece ofequipment that is made of steel, and produces high-temperature andhigh-pressure steam by reaction of a fuel. The interior of the reactorpressure vessel 110 is called the reactor core. In a boiling waterreactor (BWR) or an advanced boiling water reactor (ABWR) type ofnuclear power plant, power is generated by directly guiding steamproduced by the reactor core to a turbine. The turbine and the mainsteam pipe that guides the steam are not shown in FIG. 1.

The nuclear reactor injection equipment in an emergency core coolingsystem uses a high pressure system and a low pressure system. A highpressure system is a system that can inject a coolant into the reactorcore at a pressure even higher than the pressure in the nuclear reactor(approximately 7 MPa) during a rated operation. A low pressure system isa system for ensuring that water is injected into the reactor core afterthe pressure has been lowered by a depressurization system.

In a high pressure core flooder system (HPCF), which is one type of highpressure system, an HPCF pump (high pressure core flooder pump) 210injects water from the line indicated by D into the reactor pressurevessel 110. The water source for the high pressure core flooder systemis a condensate storage tank 250, and it is also possible to use thesuppression pool of a suppression chamber 120.

A residual heat removal system (RHR), which is one type of low pressuresystem, is a system for removing decay heat by cooling the coolant afterthe nuclear reactor has been shut down for periodic inspection or duringan emergency. This system also plays the role of one type of emergencycore cooling system (ECCS) by injecting suppression pool water into thereactor core during an emergency in order to maintain the nuclearreactor water level. The residual heat removal system (RHR) includes anRHR pump 200 and an RHR heat exchanger 202. The RHR pump 200 and the RHRheat exchanger 202 are both high-performance and are at least capable ofshifting the nuclear reactor to a cold shutdown state.

The residual heat removal system operates in multiple modes. In acontainment vessel spray mode, suppression pool water in the suppressionchamber 120 is drawn by the RHR pump 200 and cooled by being passedthrough the RHR heat exchanger 202, and then passes through the lineindicated by A and is ejected from spray nozzles 130 in the reactorcontainment vessel 100. This makes it possible to cool the interior ofthe reactor containment vessel 100 and maintain the pressure therein. Ina low pressure water injection mode, water is injected into the reactorpressure vessel 110 via a feed-water line indicated by B (nuclearreactor injecting) instead of being sent to the spray nozzles 130.Accordingly, the nuclear reactor water level is maintained at anappropriate level, and fuel overheating is prevented.

In a suppression chamber cooling mode, suppression pool water is passedthrough the RHR heat exchanger 202 and then returned to the suppressionchamber 120 via the line indicated by C. The main causes for thetemperature rise of the suppression pool water are the discharge ofsteam due to the operation of safety relief valves during nuclearreactor isolation, and the discharge of steam due to the operation ofthe reactor core isolation cooling system (RCIC). Note that duringreactor core isolation, main steam isolation valves or the like areclosed, and the nuclear reactor is isolated from the outside.

In an emergency situation in which the injection of water into thenuclear reactor by the emergency core cooling system (ECCS) is notsufficient during an accident, and there is a risk of the water level inthe nuclear reactor decreasing and the reactor core being greatlydamaged, an alternative cooling injection system (AM system) is used.

FIG. 2 is a diagram illustrating a make-up water condensate system. Themake-up water condensate system (MUWC), which is one type of alternativecooling injection system, is not originally a system for emergencies,but rather a system for supplying condensate water for the dailyoperation of equipment in buildings or the like. The water source iswater in a condensate storage tank 250, and this water is supplied usinga MUWC pump 220. When an accident occurs, water in the condensatestorage tank 250 can be supplied to the spray nozzles 130 or the reactorpressure vessel 110. If the water in the condensate storage tank 250 iscompletely depleted when an accident occurs, water can be suppliedthrough the external connection port 252 from a fire engine or the like.

However, as already described above, with an alternative coolinginjection system such as a conventional make-up water condensate systemor fire protection system, the amount of water in the containment vesselincreases. If the alternative cooling injection system operates for anextended period of time, containment vessel venting will need to beperformed.

After examination, the inventors of the present invention thought thatinstead of merely cooling the interior of the reactor containment vessel100 by injecting water, it is necessary to also cool the suppressionpool water that is already in the reactor containment vessel 100. Ofcourse the water in the nuclear reactor and the suppression pool watercan be cooled if the residual heat removal system is operating, butsituations can be envisioned in which the residual heat removal systemcannot operate due to the power supply becoming depleted ormalfunctioning, the system becoming submerged in water, or the like.Accordingly, of course the RHR pump 200 does not operate, but also thecooling capability of the RHR heat exchanger 202 is lost due to areactor building cooling sea water system (RSW) 230 not operating.

However, in order to cool the suppression pool water, it remains that itis preferable to use the RHR heat exchanger 202 in view of the pipearrangement and capability thereof.

The inventors thus focused attention on the fact that the heatexchanging of the RHR heat exchanger 202 can be implemented by analternative heat exchanger 232 such as that on a moving vehicle or thelike. The inventors then focused attention on the fact that the MUWCpump 220 can be used to supply cooled water into the reactor pressurevessel 110, and arrived at the completion of the present invention.

Specifically, in the present invention, the downstream side of the RHRheat exchanger 202 of the residual heat removal system (RHR) isconnected to the upstream side of the MUWC pump 220 of the make-up watercondensate system (MUWC). The water in the suppression chamber 120 isthus cooled by the RHR heat exchanger 202, and is injected into thenuclear reactor or sprayed in the containment vessel by the MUWC pump220.

In particular, with the present invention, circulation using thesuppression chamber 120 as the water source can be performed with use ofthe rise in pressure in the reactor containment vessel 100 caused by anaccident. However, the suppression chamber 120 is a piece of equipmentthat is underground beneath the nuclear reactor building, and is at alower position than the condensate storage tank 250 that is originallythe water source for the MUWC pump 220. For this reason, even if anattempt is made to simply draw water from the suppression chamber 120,the suctioning capability of the MUWC pump 220 will be insufficient.However, when an accident occurs, the pressure in the reactorcontainment vessel 100 rises and pushes water out from the suppressionpool, thus making it possible to ensure a necessary suction head of theMUWC pump 220. In this way, it is possible to establish a newalternative circulation cooling system that is limited to use during anaccident.

According to the above configuration, even if the emergency core coolingsystem (ECCS) that includes the residual heat removal system does notoperate normally, it is possible to suppress a rise in temperature andpressure in the reactor containment vessel 100 by cooling water from thesuppression chamber and circulating it to the reactor core. For thisreason, it is possible to suppress the implementation of containmentvessel venting due to a rise in pressure, or significantly delay theimplementation timing. In particular, the amount of water in the reactorcontainment vessel 100 does not increase when cooling the reactor core,thus making it possible to suppress a situation in which the water levelin the suppression chamber 120 rises and surpasses the venting line.Accordingly, even if venting needs to be performed, it is possible toensure the implementation of wet-well venting and avoid theimplementation of dry-well venting.

First Embodiment

FIG. 3 is a diagram illustrating a first embodiment. In the example inFIG. 3, the downstream side of the RHR heat exchanger 202 is connectedto the upstream side of the MUWC pump 220 using pipes in the highpressure core flooder system (HPCF).

The high pressure core flooder system and the make-up water condensatesystem also use the condensate storage tank 250 as the water source, andpipes directly connected to the condensate storage tank 250 are sharedby both of them. Specifically, pipes connected to the MUWC pump 220 areconnected to an intermediate portion of pipes that connect thecondensate storage tank 250 to the HPCF pump 210. In other words, pipesfor a flow in the direction opposite to the normal flow direction existbetween the upstream side of the MUWC pump 220 and the HPCF pump 210.

The high pressure core flooder system includes pipes (HPCF test line214) for circulation to the suppression chamber 120 during operationtesting of the HPCF pump 210. In other words, the high pressure coreflooder system and the residual heat removal system both have a function(mode) for circulation to the suppression chamber 120. Pipes directlyconnected to the suppression chamber 120 are shared by both of thesefunctions. Specifically, the HPCF test line 214 is connected to anintermediate portion of pipes (S/C cooling line 204) for the suppressionchamber cooling mode of the residual heat removal system. In otherwords, pipes exist between the downstream side of the RHR heat exchanger202 and the HPCF pump 210.

In this way, the downstream side of the RHR heat exchanger 202 can beconnected to the upstream side of the MUWC pump 220 via the S/C coolingline 204, the HPCF test line 214, and a line for the high pressure coreflooder system. There is no need to provide a new pipe, and the aboveconnection can be made by merely controlling the opening and closing ofvalves, thus making it possible to carry out the present inventionwithout additional construction or capital investment.

Second Embodiment

FIG. 4 is a diagram illustrating a second embodiment. In the example inFIG. 4, a bypass line 222 is provided for connecting the downstream sideof the RHR heat exchanger 202 to the upstream side of the MUWC pump 220.

In the configuration shown in FIG. 3, the HPCF pump 210 and a checkvalve 212 are provided on the line for the high pressure core floodersystem. The HPCF pump 210 creates flow resistance, and there is a riskof reducing the flow rate. Water flows backwards through the check valve212. Although the check valve 212 can be fixed in the open state byelectric control or manual control, if a large volume of water continuesto flow backwards, there is a possibility that the check valve willclose. A rise in the radiation level is anticipated at this locationwhen an accident occurs, and it is difficult to perform an on-site checkof the state of the check valve if it has closed, and then open it.

In view of this, if the downstream side of the RHR heat exchanger 202and the upstream side of the MUWC pump 220 are connected by the bypassline 222 as shown in FIG. 4, water does not flow through the HPCF pump210 or the check valve 212 that are arranged on the line for the highpressure core flooder system. Accordingly, there is no risk of the checkvalve 212 closing, and pressure loss due to the HPCF pump 210, thusmaking it possible to stably and reliably circulate water.

Although a preferred embodiment of the present invention has beendescribed above with reference to the accompanying drawings, the presentinvention is needless to say not limited to this example. A personskilled in the art will appreciate that various modifications andalterations can be made within the scope of the claims, and that allsuch modifications and alterations are also naturally encompassed in thetechnical scope of the present invention.

The present invention can be used as an alternative circulation coolingmethod for an emergency core cooling system at a nuclear power plant,and a nuclear power plant that is suited to the implementation of thesame.

What is claimed is:
 1. An alternative circulation cooling method for anemergency reactor core cooling system that is performed at a nuclearpower plant that includes a residual heat removal system (RHR) forremoving fuel decay heat when a nuclear reactor is shut down and amake-up water condensate system (MUWC) that supplies water from acondensate storage tank into the nuclear reactor, the alternativecirculation cooling method comprising: connecting a downstream side of aheat exchanger of the residual heat removal system (RHR) to an upstreamside of a MUWC pump of the make-up water condensate system (MUWC); andcooling water from a suppression chamber using the heat exchanger of theresidual heat removal system (RHR) and performing nuclear reactorinjecting or containment vessel spraying using the MUWC pump.
 2. Thealternative circulation cooling method according to claim 1, wherein:the nuclear power plant includes a high pressure core flooder system(HPCF) that supplies water from the condensate storage tank into thenuclear reactor; and the downstream side of the heat exchanger of theresidual heat removal system (RHR) is connected to the upstream side ofthe MUWC pump of the make-up water condensate system (MUWC) using pipesof the high pressure core flooder system (HPCF).